With the significant advances that have been made in drill bit technology, one of the limiting characteristics of the speed and efficiency of drilling operations is the removal of drill cuttings away from the bottom of the hole. Thus, it is important to provide the desired combination of drilling fluid flow velocity and flow volume.
One of the limiting parameters in three-cutter drill bits is the lack of space for fluid flow passage from the hole bottom to the top of the bit. The largest passageway is in the area of the nozzles between cutters but the submerged jet or nozzle stream entrains a great portion of the rising fluid and recirculates it to the hole bottom. Much of the recirculated fluid is laden with rock chips that have been removed from the hole bottom. Since there is less volume of cuttings to remove in medium and hard formations, the fluid circulation problem is not great. In soft formations with high rates of penetration, the cuttings removal problem is greatest.
Perhaps the most conventional approach is to employ three nozzles in the lower end of the drill bit between the three cutters directing three streams of high velocity fluid toward the bottom of the drilled hole. The fluid is then deflected upwardly through three spaces between the nozzle areas and the surrounding walls of the drilled hole to reach the annular space between the drill string and the drill hole. Because of the area occupied by the nozzle, the space between the nozzles and the wall of the drilled hole is somewhat restricted and the return flow tends to be equally split between the three return paths.
To enhance the horizontal or cross-flow of fluid in the bottom of the drilled hole, and thus improve the efficiency of the chip removal, some success has been had by plugging one of the three nozzles so as to cause additional cross flow from two of the nozzles towards the space adjacent the plugged nozzle.
In another prior art arrangement, two extended nozzle tubes have been used in conjunction with a third nozzle that diverts the fluid flow 180 degrees upward. The two extended nozzle tubes direct fluid flow to the hole bottom while the 180 degree curved nozzle extension directs fluid flow upwardly to increase the velocity of the fluid out of the bottom of the hole. However, the undesirable effects of this arrangement are that the 180 degree nozzle restricts the return fluid passageway and the continuous abrasive wear of the fluid through the 180 degree bend decreases nozzle durability.
Also, at least one-third of the hydraulic energy is not being expended at the hole bottom to dislodge and move rock chips with impact and lift forces. The area around the nozzles is not decreased to improve fluid passage.
In a co-pending patent application assigned to the same assignee as the present invention, one of the three nozzle passages is plugged and the nozzle cut away and replaced by a pick-up tube which extends from the lower end of the drilled hole to the space between the drill string and the surrounding walls of the drilled hole, with the purpose being to force fluid to travel horizontally along the bottom of the hole. While this arrangement shows some promise, one of its limiting characteristics is the rather small cross-section of such pick-up tube as a result of the limited space available in three-cutter drill bits.
Accordingly, a need still exists for improving the circulation of drilling fluid in three-cone drill bits.